Prevent repeatedly launching emergency updates

Emergency update of assets could be repeatedly launched
every 15 seconds if a resource could not be fetched from
a server. A cooldown period has been added to prevent
repeatedly launching emergency updates.

src/js/storage.js

@@ -1465,6 +1465,7 @@ self.addEventListener('hiddenSettingsChanged', ( ) => {
 
 {
     let timer, next = 0;
+    let lastEmergencyUpdate = 0;
 
     µb.scheduleAssetUpdater = async function(updateDelay) {
 
@@ -1477,10 +1478,13 @@ self.addEventListener('hiddenSettingsChanged', ( ) => {
             return;
         }
 
-        const assetDict = await io.metadata();
         const now = Date.now();
-
         let needEmergencyUpdate = false;
+
+        // Respect cooldown period before launching an emergency update.
+        const timeSinceLastEmergencyUpdate = (now - lastEmergencyUpdate) / 3600000;
+        if ( timeSinceLastEmergencyUpdate > 1 ) {
+            const assetDict = await io.metadata();
             for ( const [ assetKey, asset ] of Object.entries(assetDict) ) {
                 if ( asset.hasRemoteURL !== true ) { continue; }
                 if ( asset.content === 'filters' ) {
@@ -1493,8 +1497,10 @@ self.addEventListener('hiddenSettingsChanged', ( ) => {
                 const daysSinceVeryObsolete = lastUpdateInDays - 2 * asset.updateAfter;
                 if ( daysSinceVeryObsolete < 0 ) { continue; }
                 needEmergencyUpdate = true;
+                lastEmergencyUpdate = now;
                 break;
             }
+        }
 
         // Use the new schedule if and only if it is earlier than the previous
         // one.